This invention relates to filled polyphthalamide resin compositions, more particularly to improved filled compositions comprising a filled polyphthalamide, preferably a filled crystallizable or crystalline polyphthalamide, and still more particularly to filled blends comprising filler, polyphthalamide and polypropylene having improved tensile properties and rigidity together with excellent thermal processability, and to a method for providing thermally processable filled polyphthalamide compositions and composites.
The highly desirable balance of strength, stiffness and thermal properties of polyamides has gained these resins wide acceptance in the molding arts and in the extrusion arts including fiber spinning and film extrusion. The resins are found in a great variety of applications, and may be particularly attractive for use where resistance to chemical and thermal attack is required. Further, though the flexibility of these resins makes them highly desirable for many uses, where a greater rigidity is desired the resin will be compounded with reinforcement, for example with glass fiber or the like, to provide the needed increase in rigidity.
Crystalline polyamides comprising at least about 50 mole % aliphatic terephthalamide units exhibit particularly good thermal, strength and stiffness properties, and low water absorption relative to nylon 6,6. Crystalline and crystallizable polyphthalamide resins are known in the art, and a variety of grades are readily available from commercial sources, including for example the Amodel.RTM. Polyphthalamides from Amoco Performance Products, Inc. Such resins have relatively high melting points, generally about 290.degree. C. or higher, and the temperature of onset of thermal degradation for many does not greatly exceed their melting points, making the thermal processing of these resins a more rigorous and complex task, particularly for filled and reinforced polyphthalamides, than is encountered for lower melting polyamides including nylon 6,6, melting at about 260.degree.-265.degree. C. Methods for improving the processability of these high-temperature polyphthalamide resins are clearly needed by the art.
Considerable effort has been made over the years to develop methods for improving the mechanical properties of high-temperature polyamides. For example, polyamides are deficient in impact, particularly when dry, and blends with polyolefins have been developed in efforts to improve impact. However, unmodified polyamides and polyolefins are generally incompatible, and the incompatibility may cause the blends to be generally poor in important mechanical properties, and even cause partial or complete loss of integrity for articles molded or extruded from such blends. The incompatibility of polyolefins with polyamides has been overcome in the art by modifying the polyolefin component to obtain stable dispersions of polyamide and polypropylene. See U.S. Pat. No. 4,988,764. However, the addition of low modulus resins such as polyolefins tends to lower the overall rigidity and reduce performance at elevated temperatures.
The addition of fillers to polyamides, particularly reinforcing fillers such as glass fiber, has long been used to increase rigidity, and glass fiber-reinforced polyamide resins are particularly well known and widely sold for uses requiring enhanced rigidity and improved dimensional stability, particularly at elevated temperatures. The addition of glass fiber or other filler to polyamides generally provides little improvement in other mechanical properties where adhesion to the filler surface is poor.
Blends comprising polyamides and polyolefins may also be subject to reduced performance because of poor adhesion between the resin and filler. Modification of the polyolefin component with unsaturated carboxylic acid compounds and treatment of the fiber with coupling agents to provide a chemical interaction between the modified polypropylene and the fiber surfaces are among the methods that have been employed for overcoming these deficiencies, though with varying degrees of success. Such modifications frequently reduce the attractiveness and visual appearance of the surfaces of molded articles, making further efforts necessary to overcome these added problems. For example, in U.S. Pat. No. 4,613,647 there is described a method for improving the compatibility of glass reinforced, carboxyl-modified polypropylene by the addition of polyamides such as nylon 6 or nylon 6,6. The inherent reactivity of the polyamide with the carboxyl-modified polypropylene is said to cause yellowing and delustering in molded products. The patent further discloses overcoming these added problems through the use of meta-xylylene adipamides as the polyamide component of such compositions; even with these modifications, the resulting glass-reinforced blends remain somewhat deficient in rigidity and tensile properties. The addition of further additives including dyes and pigments tends to further reduce impact properties. In U.S. Pat. No. 4,657,952, there is described an improved method for combining polyamide, olefinic rubber impact modifiers and glass fiber reinforcement to provide reinforced formulations having dyes and pigments while retaining good impact properties.
A great variety of modifiers and additives that may be found useful for overcoming one or more deficiencies have resulted from these efforts, though improvement in a single resin property is quite often accomplished only at some sacrifice in the overall balance of properties. For example, although a variety of carboxylated polyolefins are known for use in combination with glass fiber-filled polyamides, in most instances such additives effect a lowering of rigidity and increase the susceptibility to thermal oxidation. Blends comprising polyamides with crystalline polyolefins or a derivative thereof are known. In U.S. Pat. No. 4,383,084 there are disclosed blends of amorphous polyamides comprising aliphatic terephthalamide and isophthalamide units with up to 10 wt % crystalline polyolefin such as high- or low-density polyethylene or polypropylene having improved impact strengths. According to the teachings of U.S. Pat. No. 3,388,186, compositions comprising crystalline nylon 6 modified with ethylene copolymers having a minor amount of acrylic or methacrylic acid or an ester derivative and grafted polycaproamide side chains are improved in impact resistance, as are similar compositions with ethylene-acrylic or ethylene-methacrylic copolymers, according to U.S. Pat. No. 3,472,916.
While the art thus discloses the blending of polyamides with a variety of polyolefins and modified polyolefins to obtain improvement in selected properties, combining filled, high-temperature, crystalline polyphthalamides with selected polyolefins or modified polyolefins to achieve improved thermal processability does not appear to have been suggested or recognized in the art.
Polyphthalamide resin formulations with improved thermal processability and moldability, particularly when filled, are continually sought by the art for use in the production of molded and extruded goods. Compositions that are able to accommodate high levels of fiber reinforcement without a concomitant loss of processability could find wide acceptance in the plastics art, and such compositions would be particularly attractive for use in demanding environments where elevated temperatures and corrosive conditions may be encountered.